A known method of fabricating aircraft parts out of composite material requires a step of using the effect of a vacuum to infuse a resin through a preform comprising fibers. More precisely, the resin is spread over a surface under the entire textile preform by using a medium that presents high permeability to the resin in comparison with the preform. Thereafter the resin is caused to infuse through the preform in a direction perpendicular to the plies, i.e. to the layers of reinforcement making up the preform. Infusion is stopped either when resin stops flowing through the inlet to the mold, or else when the quantity of resin that has been injected is the amount that is needed theoretically, which quantity is measured in terms of its weight. That relates to a method that makes use of a semi-permeable membrane. With such a membrane, the resin remains confined within the mold under the membrane. In the absence of such a membrane, the end of the injection operation is detected by resin exiting through the vents that were used to generate the vacuum in the mold.
The quantity of resin injected while implementing the method influences the soundness of the part. If the quantity that is injected is not sufficient, the part runs the risk of presenting one or more dry zones. The quantity also has an influence on the thickness of the part if the infusion is performed under a vacuum bag: it is then the volume fraction of fibers in the part that suffers and consequently the mechanical properties of the part.
Unfortunately, while making a part out of composite material by infusing resin into a textile preform placed under a semi-permeable membrane, it is difficult for various reasons to detect when the preform has finished being filled with the resin.
Firstly, it is difficult to detect when the flow of resin has stopped since measurement by weighing the injection vessel is not sufficiently accurate. The presence of pipework between the vessel and the mold disturbs the measurement. Furthermore, it is difficult to measure large volumes of resin accurately. Finally, leaks may occur in the resin circuit, in the mold, or in materials situated in its environment.
Furthermore, it is difficult to calculate the quantity of resin that needs to be injected into the preform, since it is difficult to determine the volume of the preform. Likewise, there is no control over the inside volume of the resin feed pipes, since their length is determined by the operator as a function of the setup and as a function of the size of the heater means used for polymerizing the resin after infusion. Finally, there remain uncertainties concerning the physical characteristics of the resin (coefficient of expansion, density, compressibility).
Furthermore, when infusing under a vacuum bag, i.e. not in a closed mold, it is found that the flow rate of the resin stops initially in temporary manner. When the theoretical quantity of resin has been injected, the flow rate tends towards zero, thereby reducing head losses between the injection pot and the top of the preform. The head loss is proportional to the square of the flow rate. A consequence of reducing head loss is to reduce the pressure difference between the injection pot and the top zone of the preform. If the pressure in the injection pot is equal to atmospheric pressure (as is generally true when injecting under a vacuum bag), the pressure in the preform, and thus under the vacuum bag, becomes close to atmospheric pressure. This leads to a reduction in the compacting pressure (which pressure is equal to the difference between the pressure under the vacuum bag and the pressure acting thereon and equal to atmospheric pressure). This results in an increase in the thickness of the textile preform and thus to additional resin being drawn in, thereby giving rise to a temporary increase in the flow rate before it drops once more down to zero.